The present invention relates to the field of stepper motors and, more particularly, to a multi-phase stepper motor having an outer rotor adapted for directly driving a vehicle wheel.
In response to environmental and geopolitical problems, it has become of some urgency to make available motor vehicles consuming relatively less fossil fuel than current fossil-fuel vehicles. While the ultimate target remains xe2x80x9czero-emissionsxe2x80x9d vehicles that utilize virtually no fossil fuel, realization of this goal appears far-reaching because of lack of progress in development of batteries capable of propelling a vehicle for times, distances, and speeds the driving public has come to expect.
It is now believed that in the interim, hybrid vehicles utilizing a combination of fossil-fueled engines and electric drive means may well be the answer, falling short of xe2x80x9czero-emissions,xe2x80x9d but nevertheless greatly increasing the distance one can travel on a gallon of fossil fuel. One such concept includes providing an electric motor for each wheel, the motors being driven by a generator, which, in turn, is driven by a fossil-fuel engine. In such an instance, the engine can be run solely for the benefit of the generator and therefore, can be small and run at a constant, most efficient speed, greatly reducing fossil fuel requirements. To reduce the fossil fuel requirements further, it is necessary that the wheel motors be as efficient as possible.
Two types of rotary electrical motors have been heretofore consideredxe2x80x94axial permanent magnet motors and radial permanent magnet motors. Numerous examples of both types appear in the prior art. Typically, axial permanent magnet motors feature a stator disk, or drum, with a central opening and electrical conductor windings wound through the central opening and across the outer peripheral edge of the stator disk. The stator disk typically is fixed in place. A rotor is usually mounted on a shaft near the stator disk. The rotor is provided with permanent magnets extending radially from its center.
In operation, a polyphase alternating electrical current passed through the windings of the stator disk creates a magnetic flux, to which the permanent magnets of the rotor respond, thereby turning the rotor, the shaft to which the rotor is fixed, and the vehicle wheel. The portion of the windings overlying the outer peripheral edge of the stator disk does not accomplish useful work. The magnetic field generated by the windings on the outer peripheral edge of the stator is not coupled with any of the permanent magnets in the rotor and is therefore wasted.
In general, in radial permanent magnet motors, the stator is annularly shaped and is concentrically disposed around a generally cylindrically shaped rotor. The stator is provided with electrically conductive windings wound about and in between teeth, which extend radially inwardly from the stator toward the rotor. Portions of the windings (xe2x80x9cend-turn wirexe2x80x9d) extend around the outer periphery of the stator. The rotor is provided with permanent magnets of alternating polarity disposed around the periphery of the rotor. The permanent magnets of the rotor react to a magnetic field created by current through the stator windings to cause the rotor to turn. The rotor generally is connected to a shaft, which turns with the rotor and accomplishes work. In the radial motor, the end-turn wires of the stator are not useful in creating the magnetic field that couples with the rotor permanent magnets.
Thus, there is a need for an efficient, electrically powered motor-wheel for vehicles and there is a need for more efficient motor components for use in such a vehicle wheel.
Stepper motors have been utilized since the 1940s to provide readily controllable, angular motion for a wide variety of applications. Heretofore, stepper motor designs were incapable of providing the necessary torque and electrical efficiency necessary for direct-drive vehicle wheel applications. In addition, stepper motors have required complex control and driver systems, which have also limited their application to direct-drive wheel motor applications. The inventive motor and controller designs, however, overcome these torque, efficiency, and control limitations of the prior art and provide a simple, low-cost, high-efficiency, high torque motor which is easily controlled and driven. The inventive motor design is optimized for direct-drive vehicle applications.
Several attempts at designing electric, direct-drive wheel motors for vehicles have been made.
U.S. Pat. No. 5,584,902 for SELF-PROPELLED WHEEL FOR WHEELED VEHICLES, issued Apr. 20, 1999 to Chahee P. Cho teaches one such motor. CHO, however, teaches an axial, permanent magnet motor having a stator drum fixed to an axle and a wheel hub and rim rotatably mounted to the axle. Permanent magnets are fixed on the wheel and oppose an outer lateral face of the stator drum. Excitation of the stator drum causes the permanent magnets, and thereby the wheel, to rotate. The CHO motor, while having a similar function to the stepper motor of the invention, has a totally different rotor (i.e., wheel) and stator structure, and is not a stepper motor. In addition, neither a sensor system (i.e., concentric rings of sensors) nor a detented parking brake arrangement are shown or suggested in CHO.
Another motor similar to the CHO motor is taught in U.S. Pat. No. 5,509,492 for DRIVE INCLUDING AN EXTERNAL ROTOR MOTOR, issued Apr. 23, 1996 to Bernd Pfannschmidt. The PFANNSCHMIDT motor, while serving a similar function to the inventive motor is still not a stepper motor. Like CHO, PFANNSCHMIDT neither teaches nor suggests either a sensor system (i.e., concentric rings of sensors) or a detented parking brake arrangement.
None of the prior art, individually or taken in combination, is seen to anticipate or suggest the stepper motor of the present invention.
In accordance with the present invention, there is provided an external rotor stepper motor consisting of four individual, independently controlled, co-planar stepper motors optimized for vehicle direct wheel drive applications. The innovative design eliminates the need for bifilar coil winding, each of the motor coils being wound with a single conductor, each coil also being wound in the same direction throughout the entire motor. The space between pole pieces of the motor allows the individual coils to be prewound and installed on the motor pole pieces during motor assembly. Each of the four independent motors is simultaneously energized.
Because adjacent, outside coils of adjacent motors are simultaneously energized at the same polarity, there is substantially no energy wasting cancellation of magnetic flux. Carefully chosen radial spacing of the four independent, co-planar motors also eliminates substantially all flux interaction between the motors. Because similar coils of the four motors are all connected in parallel, the net inductance presented to driver circuits is minimized. This yields fast rise time pulses and improved efficiency through minimized reactance of the motor. The physical dimensions of the motor have been chosen to provide a motor suitable for directly driving a vehicle wheel.
It is therefore an object of the invention to provide an external rotor stepper motor for direct drive of a vehicle wheel.
It is another object of the invention to provide an external rotor stepper motor having multiple, individual, independent coplanar stepper motors.
It is a further object of the invention to provide an external rotor stepper motor in which all coils are wound in the same direction for manufacturing economy.
It is an additional object of the invention to provide an external rotor stepper motor having single conductor (i.e., non-bifilar) coil windings.
It is another object of the invention to provide an external rotor stepper motor having coils that may be pre-fabricated and slid onto the stator during assembly.
It is a still further object of the invention to provide an external rotor stepper motor having concentric rings of sensors disposed in the external rotor to control drive to the stepper motor.
It is another object of the invention to provide an external rotor stepper motor having concentric rings of sensors disposed in the external rotor to allow smooth starts of the stepper motor by selecting an optimum initial energization for the stepper motor.
It is yet another object of the invention to provide an external rotor stepper motor having a plunger and detent to mechanically hold the rotor of the stepper motor when the motor is deenergized.
It is a still further object of the invention to provide an external rotor stepper motor having a controller using lead angle modulation to maximize the torque of the stepper motor as a function of motor speed.
It is another object of the invention to provide an external rotor stepper motor controller that recaptures kinetic energy from the stepper motor during braking and coasting of the motor.
It is an additional object of the invention to provide an external rotor stepper motor that may be stacked into multi-motor units for increased power output.
It is a further object of the invention to provide an external rotor stepper motor wherein a stack of the motors may be used to provide a central power plant for a motor vehicle.